The present invention relates to a method for stabilizing nitrile resins against discoloration resulting from thermal degradation during processing. More particularly this invention relates to the stabilization of polymers and graft polymers of acrylonitrile to prevent thermal discoloration by the inclusion of a minor amount of a half-acid salt of maleic acid.
Acrylonitrile polymer and copolymer resins are generally regarded as being materials of good thermal stability. They are, however, subject to thermal discoloration, and thermal processing of the unstabilized resins frequently results in development of yellow, amber and even reddish casts during the extrusion of films and fibers. Consequently a substantial effort has been made to develop methods for reducing or eliminating thermal discoloration to produce clear, color-stable films and molded articles. Prior art methods for stabilizing acrylonitrile resins against thermal discoloration include such processes as adding small amounts of alkali or alkaline earth metal salts of weak acids as stabilizers, such as is shown in U.S. Pat. No. 3,527,730. More recently, the addition of minor amounts of salts of half esters of maleic or fumeric acid to acrylonitrile resins has been shown to provide improved stability against thermal discoloration, such as is shown in U.S. Pat. No. 3,984,499.